The present document is based on published International Patent Application No. WO 99/29005, the entire contents of which being incorporated herein by reference.
1. Field of the Invention
This invention relates to a power cable termination for connecting cryogenic high voltage apparatus to a room temperature high voltage line. In particular, but not exclusively, the invention relates to a superconducting power cable termination, such as a termination for a high-temperature (high-Tc) superconducting power cable. The invention also relates to a power cable provided with such a termination and electrical apparatus, such as a power transformer or generator, provided with such a termination. The cable termination is intended to be able to deal with very high operating voltages, e.g., up to 800 kV or more.
2. Discussion of the Background
There are two main functions of a termination for a superconducting power cable. Firstly there is the requirement for converting the high radial electric field in a superconducting cable to an axial electric field after the termination. Secondly, there is the need for the termination to be able to provide the transition between room and cryogenic temperatures. A third requirement is for the termination to be designed for high voltages.
Development work on a termination for a high-Tc superconducting (hereinafter referred to as HTS) cable is described in an article entitled xe2x80x9cDevelopment of Termination for the 77KV-Class High Tc Superconducting Power Cablexe2x80x9d by T. Shimonosono, T. Masuda and S. Isojima in IEEE Transaction on Power Delivery, Vol. 12, No. 1, January 1997. The main disadvantage of terminations of this known type is that such terminations use liquid nitrogen both as a coolant and as a dielectric. Nitrogen gas bubbles are produced due to the heat inlet and joule heat and these nitrogen bubbles are believed to cause breakdown of the current lead of the termination at increased power levels.
An aim of the present invention is to provide a termination for a cryogenically cooled power cable which overcomes the problems of breakdown associated with known terminations.
A further aim of the present invention is to dispense with the use of liquid nitrogen for electrical insulation of the termination.
According to one aspect of the present invention there is provided a power cable termination having a current lead, a power cable having an inner first tube and an outer conductor, whose electrically conducting properties improve at low temperatures, arranged around the first tube and intended in use to be cooled to low temperatures by cryogenic fluid flowing through the first tube, a joint electrically connecting one end of the current lead to the conductor at one end of the cable at or adjacent to one end of the first tube, and a second tube communicating with the first tube at or adjacent to the joint for conveying cryogenic fluid to or from the first tube, the first and second tubes being arranged so that, in use, no cryogenic fluid conveyed by the tubes contacts the conductor or the current lead at the joint.
In use of the termination, the cryogenic fluid, e.g. liquid nitrogen, conveyed through the first and second tubes acts solely as a cooling medium for the conductor and does not serve as an electrically insulating medium at the joint. Thus a different medium can be used to provide electrical insulation of the conductor and the current lead at the joint.
In most practical applications, the conductor has superconducting properties. However, the invention is not intended to be limited to conductors having superconducting properties and is intended to cover any conductor whose electrical conducting properties significantly improve at low temperatures, e.g. at temperatures below 200 K, preferably below 100 K, e.g. 77 K. In the preferred case of the conductor having superconducting properties, the conductor may include low temperature semiconductors but preferably has a high-Tc superconductor. For example the high-Tc superconductor may be silver sheathed BSCCO wire or tape, such as BSCCO-2223 (where the numerals indicate the number of atoms of each element in the [Bi, Pb]2Sr2Ca2CU3Ox molecule) or BSCCO-2212. Other examples of known HTS tapes are TiBa2Ca2Cu3Ox(TBCCO-1223) and YBa2Cu3Ox(YBCO-123).
The power cable suitably has a main portion in which the conductor is surrounded by electrical insulation, e.g. of solid polymeric material, which, in use of the cable, provides a radial electric field contained within the surrounding electrical insulation, a cable terminating device spaced from said one end of the power cable for converting, in use of the cable, the radial electric field to a substantially axial electric field, and an end portion extending from the cable terminating device to the said one end of the power cable. Conveniently the joint has a high electric potential metallic corona shield to which the conductor lead and the conductor are connected, e.g by soldering.
Preferably, the electrical insulation surrounding the conductor has an inner layer of semiconducting material which is electrically connected to the conductor and an intermediate layer of electrically insulating material which surrounds the semiconducting inner layer. The said main portion of the cable also has an outer layer of semiconducting material, which is connected to a controlled electric potential, preferably earth potential, along its length, and which surrounds the said intermediate layer of electrically insulating material. This semiconducting outer layer is not present along the length of the said end portion of the cable, e.g. it is removed to reveal the underlying intermediate layer.
In this specification the term xe2x80x9csemiconducting materialxe2x80x9d means a substance which has a considerably lower conductivity than an electric conductor but which does not have such a low conductivity that it is an electric insulator. Suitably, but not exclusively, the semiconducting material will have a resistivity of from 1 to 105 ohmxc2x7cm, preferably from 10 to 500 ohmxc2x7cm and most preferably from 10 to 100 ohmxc2x7cm, typically 20 ohmxc2x7cm.
The intermediate layer preferably includes a polymeric material such as, for example, low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), cross-linked materials such as cross-linked polyethylene (XLPE) or rubber insulation such as ethylene propylene rubber (EPR) or silicone rubber. The semiconducting layers are formed of similar polymeric materials but with highly electrically conductive particles, e.g. carbon black or metallic particles, embedded therein. Typical examples of materials for the insulating and semiconducting layers are disclosed in U.S. Pat. No. 4,785,138.
Preferably a string of axially arranged annular insulating elements, e.g. of porcelain, glass, polymeric material or rubber material, such as silicone rubber or EPR, surround the said end portion of the cable and extend between the cable terminating device and the joint. The annular insulating elements prevent creepage along the outside of the electrical insulation of the end portion of the cable.
In a first design of termination, the superconductor is arranged around the first tube but not around the second tube which is intended to connect the first tube to cryogenic fluid cooling apparatus. The second tube may be led directly away from the termination at the joint. Alternatively, however, the second tube may be positioned back along or around the outside of the superconductor so as to extend back from the joint towards the cable terminating device inside the string of annular insulating elements before being led away from the termination. In this case, the second tube is preferably wound around the layer of superconductor surrounding the first tube. Preferably solid thermal insulation, e.g. of polymeric material having an electric field stress xe2x89xa60.2 kV/mm, is positioned between the second tube and the surrounding string of annular insulating elements. The portion of the second tube leading away from the termination is preferably surrounded by thermal insulation.
According to a second design of termination, the first and second tubes have coolant supply and return ducts of a central coolant dusting member of the power cable around which the conductor, e.g. superconductor, in tape or wire form, is helically wound. Thus the conductor is wound around both the first and second tubes. The central coolant ducting member may be divided internally to provide said first and second tubes, the first and second tubes communicating with each other at the said one end of the first tube. In its simplest form, the internal division of the central coolant ducting member is provided by a diametric partition wall. However to improve thermal efficiency and to ensure bending in all directions, the ducking member and/or the internal partition wall, may be helically twisted. As an alternative to internally dividing the central coolant ducting member, the latter may be formed as a single tube having a return bend portion at the joint connecting the first and second tubes which convey the cryogenic fluid in opposite directions in the power cable.
According to other aspects of the present invention there is provided a power cable and electrical apparatus provided with a termination according to said one aspect of the present invention.